The Brighton triathlete was on his way home after coming to the Fens to pick up a bike he had bought from ebay when he saw the sinking Skoda Citigo on the A1122 on Saturday at about 5pm.

Ray Matthews, who stopped when he saw a commotion at the side of the road, said: “It was really surreal - one minute I was driving along a beautiful country road having collected a racing bike bought from ebay and the next minute I had jumped into water and was grappling with a door to get an old man out of his car.

“I think he must have had a stroke at some point in the past as he said his left arm had no feeling and he couldn’t undo his seat belt.

“The water was rushing in to the car and it was sinking pretty quick. His window was half open and I managed to yank the door open.

“I then had to duck my head under water to undo his seat belt.

“Luckily another motorist jumped into the water and helped me drag him to the side of the riverbank where his wife was. I think she was in shock.

“I can only assume he had parked up and was going to start up again and had accidentally put his foot on an accelerator instead of a brake and ended up in the water.”

Mr Matthews, 48, then drove to the Lamb and Flag pub at Welney where he changed into a set of runners clothes that, being a keen athlete, he always keeps in the boot of his car.

“It was the most surreal experience, walking into a village pub soaking wet,” said Mr Matthews, who runs his own curtain and blind upholstery business in Brighton.

Norfolk Police and the Fire and Rescue Service confirmed the old man did not need to be taken to hospital.

A Norfolk Police spokesman said it remained a mystery as to why the car had ended up in a water-filled ditch.

The spokesman said: “The ditch is quite deep and the car was partially submerged. Luckily there were no injuries and they were taken home while the vehicle was pulled out of the water.”